Coaxial cable with helically wound insulating spacer



Oct. 27, 1964 R. c. MILDNER 3,154,629

COAXIAL. CABLE WITH HELICALLY WOUND INSULATING SPACER Filed Sept. 5, 1961 FIG'Z INVENTOR RAYMOND CHARLES MILDNER ATTORNEY United States Patent 3,154,629 COAXIAL CABLE WITH HELICALLY WOUND INSULATING SPACER Raymond C. Mildner, 3703 Hillgrove Court, Midland, Mich. Filed Sept. 5, 1961, Ser. No. 136,070 a Ciairns. (or. 174 2a) This invention relates to coaxial cables.

In air-spaced, and semi-air-spaced coaxial cables it has been proposed to employ a helical form of insulating structure betweenthe inner and outer conductors, because this form of insulation does not impose any serious obstacle to the transmission of electrical energy at the operational frequencies of such cables.

In the specification of my British Patent No. 608,147 I have proposed a helical insulating structure in which the helix is, in effect, cut from a thick-walled tube of insulating material. Another proposal has been to form the helix from a number of accurately superposed thin insulating tapes, and another proposal has been to lap a thermoplastic tape of rectangular cross-section helically around an inner conductor, whereafter the tape is deformed by heat so that after cooling it is mechanically stable in the helical form. In the application of this last proposal to larger cables, triangular section prisms may be cut at regular intervals from the inside of the tape to lessen the distortion which the tape has to undergo in order to adjust its length to correspond to the length of the different helical parts corresponding to positions ranging from the inner to the outer conductors.

In all of these designs, the limitation which prevents the thickness of the helix from being reduced, which is desirable in order to improve the transmission efiiciency and to increase the velocity of propagation, is the stability of the helix against crushing and lateral thrust. Each of the constructions described above is a helix of substantially rectangular cross-section, which is not the form best suited to provide stability against crushing and lateral movement.

According to the present invention there is provided in, or for use in a coaxial cable, an insulating structure lying, or adapted to lie between the inner and outer conductors, the insulating structure comprising at least one strip or tape of insulating material corrugated for at least a marginal portion thereof, the arrangement of the insulating structure being such that when it is in position in the coaxial cable, it extends helically around the inner conductor and lies edge-on thereto, with the corrugated marginal portion lying adjacent the inner conductor and following a substantially sinusoidal path in a plane normal to that of the strip or tape when uncorrugated.

The outer edge of the strip or tape may follow a substantially helical path, in which case the parts of the tape intermediate between the two edges follow substantially sinusoidal paths having amplitudes which increase as the inner edge is approached. Alternatively, the whole of the strip or tape may be corrugated, in which case the outer edge of the tape may follow a substantially sinusoidal path, the amplitude of which is less than that of the inner edge of the tape. By corrugating the tape in this manner, the length of the tape is adapted to the various coaxial helical paths that lie between the outer and inner conductors.

Preferably the insulating structure comprises two strips or tapes corrugated and joined together, or one corrugated tape which is folded in half longitudinally.

The insulating tape is formed of any conventional insulating material, such as polyethylene, or polypropylene.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings, which illustrate diagrammatically and by way of example some embodiments thereof, and in which: 7

FIG. 1 is a perspective view of structure for a coaxial cable,

FIG. 2 is an elevational view of part of a coaxial cable showing the insulating structure in position, and

FIG. 3 is a view similar to FIG. 2 and showing a further insulating structure in position.

Referring now to FIG. 1, two tapes 1 and 2 of synthetic plastic material are corrugated along their length and laid adjacent one another so that the corrugations are out of phase. The two tapes are then united to one another along the lines of contact which occur every 360 of the sine wave formed by the corrugations.

The composite tape so formed is then wound edgewise around the inner conductor 3 of the coaxial cable shown in FIG. 2, with a sufficient tension to straighten out or substantially to straighten out the corrugations of the outside edge 4, and to increase the amplitude of the corrugations of the inside edge 5.

The edges of the tapes which are to form the inner edge 5 of the insulating structure are sinusoidally scalloped so that when the tape is wound edgewise around the inner conductor 3 the inner edge 5 of the tape will at all points contact the inner conductor. For stability, it is important that contact with the inner conductor should at least be made at the points where the two walls of the insulating structure are most widely separated.

Furthermore, fine rods of rigid insulating material may be inserted across the tapes as indicated at 6 by broken lines in FIG. 2, so as to be trapped between the tapes at the points of contact thereof. The rods may be of quartz, ceramic or other material having comparable electrical part of an insulating V and mechanical properties, and serve to support the inner conductor radially, especially at high temperatures when the synthetic plastic material of which the insulating structure is formed and which is thermoplastic may be little more than form stable.

Referring now to FIG. 3 of the drawings there is shown an insulating structure for use with an inner conductor '7 which is helically corrugated with a two-start helical corrugation comprising the two corrugations 8 and 9. The insulating structure is formed from a single tape 10 which is folded in half longitudinally and corrugated. The tape is then wound edgewise onto the inner conductor so that each of the corrugated edges follows one of the separate grooves of the two-start helical corrugations on the inner conductor. The accuracy with which the insulating structure can be positioned is achieved by the corrugations of the inner conductor in which the edges of the tape lie, and the stability of the insulating structure is ensured by its triangular form and the transverse corrugations.

In a modification of the structure shown in FIG. 3, a single tape is folded in half longitudinally, and corrugations are formed in the tape in such a manner that the corrugations have a maximum amplitude at the free edges. Again the corrugations are formed 180 out of phase and are welded together at the points where they contact.

Having now described the invention, what is claimed as new and desired to be secured by Letters Patent, is:

1. A coaxial cable comprising inner and outer conductors and a helically wound insulating tape in between and contacting the conductors, said tape being corrugated, the edge of the insulating tape contacting the inner conductor being additionally scalloped so that at least the points of the inner edge having the maximum amplitude of corrugation contact with the inner conductor.

2. A coaxial cable as defined in claim 1, wherein the a r r as parts of the tape intermediate the two longitudinal edges of the tape follow substantially sinusoidal paths having amplitudes which increase as the inner edge is approached. 3. A coaxial cable as defined in claim 1 wherein the outer edge of the tape follows a substantially undistorted sinusoidal path. v

4. A coaxial cable as defined in claim 1 wherein the tape comprises two members, each corrugated, with the corrugations 180 out of phase and the members being united to each other along the lines of contact.

5. A coaxial cable as defined in claim 4 in which fine rods of rigid insulating material extend across the width of the members for reinforcing the structure.

6. A coaxial cable as defined in claim 1 in which the r 4 7. A coaxial cable as defined in claim 1 wherein the inner conductor has a two-start helical corrugation surrounding the same, said tape comprised of a single strip of material folded in half longitudinally with the free longitudinal edges of the tape engaging the inner conductor at the junctures of said corrugations with the outer surface of the inner conductor.

8. A coaxial cable as defined in claim 7 wherein the free longitudinal edges of the tape engage the junctures of both of the corrugations with the inner conductor.

References Qited in the file of this patent UNITED STATES PATENTS 1,885,168 AfI'el Nov. 1, 1932 

1. A COAXIAL CABLE COMPRISING INNER AND OUTER CONDUCTORS AND A HELICALLY WOUND INSULATING TAPE IN BETWEEN AND CONTACTING THE CONDUCTORS, SAID TAPE BEING CORRUGATED, THE EDGE OF THE INSULATING TAPE CONTACTING THE INNER CONDUCTOR BEING ADDITIONALLY SCALLOPED SO THAT AT LEAST THE POINTS OF THE INNER EDGE HAVING THE MAXIMUM AMPLITUDE OF CORRUGATION CONTACT WITH THE INNER CONDUCTOR. 